Mollusca

• 1. The Soft Ones

  ›

  Mollusca is a phylum of soft-bodied invertebrate animals, and 86,600 living species of them have been recognized. That makes it the second-largest animal phylum, behind only the arthropods. The proportion of undescribed living species is very high, and many groups remain poorly studied. Estimates of fossil species run between 60,000 and 100,000 more.

  The name itself records an old idea about softness. It comes from the French mollusque, which traces back to the post-classical Latin mollusca, drawn from mollis, meaning soft. The naturalist J. Jonston first used it in his Historiae Naturalis of 1650 to describe a group of cephalopods. In classical Latin, molluscus described only a particular type of soft nut.

  Aristotle may have shaped this choice. He wrote of ta malakia, the soft ones, from malakos for soft, a label he applied among other things to cuttlefish. The scientific study of molluscs is called malacology, an echo of that ancient word. But how does a single phylum hold the snail, the clam, the octopus, and a creature shaped like an elephant's tusk? What unites animals so unalike, and where did they come from? A backward-pointing cone of feces inside the stomach of a snail will turn out to matter more than it sounds.

• 2. A Hypothetical Ancestor

  ›

  Finding defining characteristics that apply to every modern mollusc is difficult, because the phylum has developed such a varied range of body structures. So many textbooks describe an imagined creature instead. They call it the hypothetical ancestral mollusc, and its depiction looks rather similar to modern monoplacophorans.

  This generalized mollusc is unsegmented and bilaterally symmetrical, with a single limpet-like shell on top. The shell is secreted by a mantle that covers the upper surface, and the underside consists of a single muscular foot. The visceral mass, the soft nonmuscular metabolic region, holds the body organs.

  Four features turn up most universally across living molluscs. There is a soft body composed almost entirely of muscle, and a mantle with a significant cavity used for breathing and excretion. There is a radula, a rasping tongue, which is absent only in the bivalves. And there is the particular structure of the nervous system.

  Many shells share a common architecture of three layers. The outer periostracum is made of organic matter, the middle layer of columnar calcite, and the inner layer of laminated calcite that is often nacreous. Molluscs never use phosphate to construct their hard parts, with the questionable exception of Cobcrephora. The first mollusc shell almost certainly was reinforced with the mineral aragonite.

• 3. The Rasping Tongue

  ›

  The radula is unique to the molluscs and has no equivalent in any other animal. It is a muscular tongue bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. Its primary job is to scrape bacteria and algae off rocks, and it works alongside a cartilaginous supporting organ called the odontophore.

  Mucus does much of the quiet work of feeding. Glands in the mouth secrete a slimy mucus to which food sticks, and beating cilia drive that mucus toward the stomach as a long string. At the tapered rear of the stomach sits the prostyle, a backward-pointing cone of feces and mucus that further cilia rotate. It acts as a bobbin, winding the food string onto itself.

  Sorting happens by acidity and by yet more cilia. Before the string reaches the prostyle, the stomach's acidity makes the mucus less sticky and frees the particles. Smaller particles, mainly minerals, are sent to the prostyle and eventually excreted, while the larger ones, mainly food, go to the stomach's cecum to be digested. The process is by no means perfect.

  Not every mollusc rasps rock for algae. Cephalopods are primarily predatory, with the radula taking a secondary role to the jaws and tentacles. Sacoglossan sea-slugs suck sap from algae, using their one-row radula to pierce the cell walls. The monoplacophoran Neopilina includes in its diet a protist called the xenophyophore Stannophyllum.

• 4. Blood, Gills, And Nerves

  ›

  Hemocyanin colors the blood of most molluscs, a respiratory pigment that carries oxygen. Their circulatory systems are mainly open, the exception being cephalopods, whose systems are closed. The main body cavity is a hemocoel through which blood and coelomic fluid circulate, and these hemocoelic spaces act as an efficient hydrostatic skeleton. Although molluscs are coelomates, the coelom is reduced to small spaces around the heart and gonads.

  The heart doubles as part of the plumbing for waste. It consists of one or more pairs of atria that receive oxygenated blood from the gills and pump it to the ventricle, which sends it into a short aorta. The atria also filter waste from the blood and dump it into the coelom as urine. A pair of metanephridia, the little kidneys, then extract reusable materials before ejecting the rest into the mantle cavity. The Planorbidae, or ram's horn snails, break the pattern by using iron-based hemoglobin instead of copper-based hemocyanin.

  Gills tend to be feather-shaped, and most molluscs have only one pair or even a single gill. Their filaments carry three kinds of cilia, one driving the water current and the other two keeping the gills clean. A pair of chemical sensors called osphradia guards the incoming water. If they detect noxious chemicals, the gills' cilia may stop beating until the intrusion ceases.

  The nervous system reaches its strange extreme in the bivalves. The cephalic molluscs carry two pairs of main nerve cords organized around paired ganglia, with the pedal ganglia controlling the foot below the esophagus. Bivalves have only three pairs of ganglia, and the visceral pair serves as the principal center of thinking. Some, such as the scallops, even have eyes around the edges of their shells that distinguish light from shadow.

• 5. From Larva To Adult

  ›

  A trochophore is the most basic molluscan larva, planktonic and ringed with two bands of cilia around its equator. Those bands sweep floating food into the mouth, and more cilia drive it into the stomach and expel the remains through the anus. As new tissue grows in the interior, the apical tuft and the anus are pushed further apart.

  The veliger stage often follows. In it the prototroch, the equatorial band nearest the apical tuft, develops into the velum, a pair of cilia-bearing lobes the larva swims with. Eventually the larva sinks to the seafloor and metamorphoses into the adult form. Cephalopods skip all of this through direct development, hatching as a miniaturized version of the adult.

  Reproduction begins simply and grows complicated. The simplest system relies on external fertilization, with two gonads shedding ova or sperm into the small coelom around the heart. The nephridia then carry the gametes into the mantle cavity. Molluscs using this method stay one sex for life, while others practice internal fertilization or are hermaphrodites that function as both sexes.

  Some populations dispense with a partner entirely. C. obtusus is a snail endemic to the Eastern Alps, and there is strong evidence for self-fertilization in its easternmost populations. The development of molluscs has drawn fresh interest in the study of ocean acidification, where environmental stress is known to affect the settlement, metamorphosis, and survival of larvae.

• 6. Eight Classes

  ›

  Opinions vary about how many classes molluscs form, and the phylum is typically divided into seven or eight, two of them entirely extinct. The gastropods are by far the most diverse, accounting for about 80% of all classified molluscan species. They number some 70,000 described living species and inhabit marine, freshwater, and land environments, ranging from snails and slugs to abalone, limpets, conch, nudibranchs, and sea butterflies.

  The bivalves run a distant second with about 20,000 species, among them clams, oysters, scallops, geoducks, mussels, and the extinct rudists. The cephalopods, with around 900 species, include squid, octopuses, cuttlefish, nautiluses, vampire squids, and the extinct belemnites and ammonites. They rank among the most neurologically advanced of all invertebrates. Either the giant squid or the colossal squid is the largest known living invertebrate, the colossal surpassing the giant in weight but not in length.

  Smaller classes round out the living phylum. The Polyplacophora, or chitons, number about 1,000 species of the rocky tidal zone and seabed, their eight shell plates penetrated by living tissue carrying nerves and sensory structures. The Scaphopoda, the tusk shells, number about 500. The worm-like Aplacophora number about 320, and the Monoplacophora, an ancient lineage with cap-like shells, number just 31.

  Two classes survive only as fossils. The Rostroconchia are probable ancestors of the bivalves, and the Helcionelloida are snail-like molluscs such as Latouchella. Where these and the living classes sit on the family tree is a question scientists have been unable to settle.

• 7. A Contested Family Tree

  ›

  Kimberella, dating from the Ediacaran, has been described by some paleontologists as mollusc-like, while others will go no further than probable bilaterian, if that. The phylogeny of molluscs is a controversial subject, tangled by debates over which ancient fossils truly belong. An even sharper argument surrounds Wiwaxia, and it centers on whether its feeding apparatus was a kind of radula or something closer to that of polychaete worms.

  The helcionellids offer firmer ground. They first appear in Early Cambrian rocks from Siberia and China, with rather snail-like shells, which means shelled molluscs predate the earliest trilobites. Most helcionellid fossils are only a few millimeters long, though specimens a few centimeters long have been found, the tiny ones suggested to be juveniles and the larger ones adults.

  The earliest cephalopods crept rather than swam. Volborthella was long thought to be one, until detailed fossils showed its shell was built from grains of silica rather than secreted. The Late Cambrian Plectronoceras is now the earliest undisputed cephalopod, its shell carrying septa and a siphuncle, but it crawled along the seafloor, ballasted with stony deposits on its underside and marked with stripes and blotches above.

  Whether the phylum even holds together as a single lineage has been questioned. A 2009 analysis combining morphological and molecular data concluded the molluscs are not monophyletic, with Scaphopoda and Bivalvia as separate lineages. A 2010 analysis recovered the traditional groups and found the molluscs monophyletic, showing that earlier data for solenogastres had been contaminated. The latest studies keep adding new permutations rather than ruling old ones out.

• 8. Food, Pearls, And Venom

  ›

  Bivalves such as clams and mussels have fed humans since at least the advent of anatomically modern humans, often to the point of overfishing. In 2005, China accounted for 80% of the global mollusc catch, netting almost 11,000,000 tonnes, while within Europe France remained the industry leader. Toxins that accumulate in certain molluscs create a risk of food poisoning, which is why many jurisdictions regulate their handling.

  Luxury followed close behind food. The best natural pearls come from the marine pearl oysters Pinctada margaritifera and Pinctada mertensi of the tropical Pacific, forming when a foreign object lodges between mantle and shell. Tyrian purple, drawn from the ink glands of murex shells, fetched its weight in silver in the fourth century BC, according to Theopompus. Sea silk was woven from the byssus threads of bivalves such as Pinna nobilis, and Procopius wrote around 550 CE that the five hereditary satraps of Armenia wore cloaks of lana pinna.

  Shells served as money in some preindustrial societies. Cowries and others functioned as shell money, though these currencies often differed from government-backed money. Some were used mainly as social status displays at occasions such as weddings, while others traded as a means of exchange whose value shifted from place to place.

  Not every encounter is benign. Blue-ringed octopuses of the genus Hapalochlaena bite only if severely provoked, yet their venom kills 25% of human victims. The venom of cone snails is so precise on the nervous system that it has become a useful tool in neurological research, and the small size of its molecules makes it easy to synthesize. Schistosomiasis, carried by freshwater snail hosts, infects an estimated 200 million people in 74 countries, with 100 million in Africa alone.